Rosin esters prepared as liquid resins by esterification of rosin and monohydric or polyhydric alcohol have been heretofore recommended for use as additives, modifiers or the like for adhesives, coating compositions or the like. Especially a liquid rosin ester is used as an important constituent of a sticky adhesive because of its tackiness, namely as a liquid tackifier. Known liquid tackifiers other than the rosin ester include, for example liquid terpene resins, liquid petroleum resins, polybutenes, liquid polyisoprenes, etc. Liquid rosin esters are excellent in tackiness as compared with the above other liquid resins and process oil (process oil or the like being useful as a plasticizer) but are not invariably satisfactory in color tone and heat stability.
For the above reasons, it is desired in industries to develop new liquid resins, particularly liquid rosin esters, which are high in tackiness, light-colored, and significantly stable.
It is known that a liquid rosin ester having an improved heat stability can be produced by disproportionation or hydrogenation of rosin, followed by esterification. However, such process provides rosin esters which are colored due to the thermal degradation of rosin in the course of the reaction, and it is difficult to prevent the coloration. Further the process gives rosin esters which are short of satisfactory in the heat stability required of liquid tackifiers.
Japanese Unexamined Patent Publication No. SHO 59-230072 discloses a technique for preparing a light-colored and highly stable liquid rosin ester by subjecting to esterification reaction a rosin purified by distillation and alcohol in the presence of a specific organic sulfur compound. However, this process essentially involves two procedures, i.e. distillation and esterification procedures, and thus is low in productive efficiency and also economically disadvantageous.
In view of the present situation as stated above, the present inventors conducted extensive research to develop a novel liquid rosin ester which is fully satisfactory in any of color tone, heat stability, productive efficiency and other requirements, particularly by directing attention to a stabilizer to be added in esterification reaction. Our research revealed that when a specific alkali metal iodide and anthraquinone are used in combination, there is provided a technique for producing a liquid rosin ester which answers the foregoing purpose. The present invention has been accomplished based on this novel finding.